The present invention relates to a spark plug for use in internal-combustion engines in which a composite chip comprising a discharging layer and a thermal stress relieving layer is disposed in the discharge portion of an electrode, and a manufacture method of the spark plug for use in internal-combustion engines which is improved to enhance heat resistance and durability of the composite chip.
Engines of automobiles and the like include spark plugs of the type that a precious metal such as Pt (platinum) or a Pt alloy is disposed in discharge portions of the spark plug. This type spark plug can be used for a long period of time in maintenance-free fashion because such a precious metal is disposed in the discharging portions which are most severely worn.
Meanwhile, from the standpoint of environmental protection aiming to reduce fuel consumption and conform with exhaust gas regulations, there is a tendency in engines to increase the compression ratio and achieve leaner burning. This tendency means that the temperature in a combustion chamber of the engine is raised. In the discharge portions of the spark plug, therefore, the thermal stress becomes larger which is attributable to a difference in linear expansion coefficient between the previous metal and an electrode material of the discharge portions.
In view of the above, as disclosed in Japanese Patent Publication No. 3-22033 and Japanese Patent Unexamined Publication No. 60-262374, it has been proposed to employ a composite chip comprising a heat stress relieving layer and a discharging layer. The composite chip is welded to the discharge portion of an electrode with the heat stress relieving layer facing the discharge portion. The heat stress relieving layer has a linear expansion coefficient between coefficients of the discharging layer and the discharge portion.
Recently, however, there has been a demand for higher performance of spark plugs. This demand has increased more and more the thermal load imposed on spark plugs.
As a result, there may occur oxidation corrosion due to the heat stress at the joint interface between the discharging layer and the heat stress relieving layer which have been joined beforehand, or falling-off of the discharging layer in the extreme case, making spark plugs not durable for a long period of time.
Furthermore, if resistance welding is performed under energizing conditions suitable to provide a sufficient degree of adhesion at the joint interface when joining the composite chip to an electrode material, the heat stress relieving layer is melted, by the Joule heat generated at the joint interface between the heat stress relieving layer of the composite chip and the electrode material, to spread out as a linear burr along its outer periphery. Consequently, the heat stress relieving layer is thinned remarkably.
In addition, at the time of welding the composite chip to the discharge portion, it is required to discriminate which side of the composite chip is the discharging layer or the heat stress relieving layer.
However, the conventional composite chip has a thickness of about 0.3 to 0.7 mm and, in many cases, both the layers thereof have almost the same thickness. This makes it difficult to discriminate the discharging layer and the heat stress relieving layer from each other.
With the foregoing in mind, there has been proposed another prior art that a composite chip in the discharging layer and the heat stress relieving layer which are different in edge size, or a composite chip which is marked such as by coloring the bottom surface at either one side, is welded to the discharge portion of the spark plug (see Japanese Patent Unexamined Publication No. 60-262374).
Provision of a difference in edge size between the discharging layer and the heat stress relieving layer, however, is very difficult in a step of punching the composite chip. Accordingly, a step of providing such a difference in edge size is added separately from the step of punching the composite chip. This leads to an unreliable result because of a fear that the difference in edge size may be provided reversely by mistake.
Moreover, the addition of the new step increases the production cost. In the case of marking the composite chip by such as coloring either one side thereof, the production cost is also increased owing to the addition of a new step.